Coral mucus stable isotope composition and labeling: experimental evidence for mucus uptake by epizoic acoelomorph worms

Mucus released by scleractinian corals can act as an important energy and nutrient carrier in coral reef ecosystems, and a distinct isotopic signature would allow following the fate of this material. This study investigates the natural C and N stable isotopic signatures of mucus released by four scleractinian coral genera (Acropora, Fungia, Pocillopora and Stylophora) in comparison with those of suspended particulate organic matter (POM) in seawater of a Northern Red Sea fringing coral reef near Aqaba, Jordan. The natural delta C-13 and delta N-15 signatures of coral mucus differed significantly from seawater POM for the majority of seasonal comparisons, but were inappropriate for explicit tracing of mucus in the coral reef food web. Thus, a labeling technique using stable isotope tracers (C-13 and N-15) was developed that produced delta C-13 values of up to 122 +/- A 5aEuro degrees (mean +/- A SE) and delta N-15 of up to 2,100 +/- A 151aEuro degrees in mucus exuded by Fungia corals. C-13 and N-15-enriched compounds were rapidly (within 3 h) and light-dependently transferred from the endosymbiotic zooxanthellae to the mucus-producing coral host. The traceability of N-15-labeled mucus was examined by evaluating its uptake and potential utilization by epizoic acoelomorph Waminoa worms naturally occurring on a range of scleractinian coral taxa. This tracer experiment resulted in uptake of coral mucus by the coral-associated acoelomorphs and further demonstrated the possibility to trace stable isotope-labeled coral mucus by revealing a new trophic pathway in coral reef ecosystems.